My sister married a man after having an adulterous affair with him. They have now been married and have two children. My sister's husband has just told her he has had an affair with yet another woman who is now pregnant with his child and he has announced that he is leaving to be either alone or move in with the new woman. Sin seems to have begat sin. I think he should make amends and go back to wife number one if she will have him and his last choice should be new woman with baby. What would the Bible say?
From the history you have given me, it is not likely that he will care what the Bible says. He has shown an unwillingness to follow biblical principles in his life, and an unwillingness to change.
Nothing in the Bible says a person has to divorce the unfaithful party to the contract. If he initiates the divorce so he can be with his new girlfriend that is not a valid reason for divorce under the New Testament. On the other hand, if his current wife took him away from a previous wife, she should have expected something like this. If she was the married party in the adultery and he was unmarried, then this action was probable, but not necessarily expected.
It has long been debated whether Jesus expected or would have allowed a husband to return to a former wife in such a circumstance. Some say that a remarriage of an adulterer just continues that adultery and he should return to the former spouse. They don't consider the new marriage valid in any way, although Jesus called it a divorce (Matthew 5:31-32). Others think the new marriage may be an act of adultery, but that the marriage is still valid. In that case Jesus may have expected him not to return to his former spouse, since the Law of Moses said that if a man divorced a wife and she remarried, then was divorced or widowed, the original husband could not marry her again. (Deuteronomy 24:1-4) God calls such a remarriage an "abomination." Some have said this was to prevent wife swapping. Whatever its reason, it prevented remarriage if another marriage intervened.
Ideally, the guilty party would return to the spouse, and all would be forgiven and life go on. Ideally that individual would not be unfaithful again. In this case the track record doesn't make that look promising. Ideally, also, no children would be involved. Unfortunately, there is no good solution for the children if they are from two different families.
The whole debate may, in most cases be meaningless because one or both parties obviously ignore the scriptures anyway. It often only becomes important if the adulterous party later becomes a Christian. In that case, though, his former sins are forgiven and it may not be a matter of concern any more.